Neutronic reactor fuel element and method of manufacture



Jan- 24, H, M FINNISTON HAL NEUTRONIC REACTOR FUEL ELEMENT AND METHOP OF MANUFACTURE Filed Feb. 21, 1950 1N1 EN TORS HAROLD MONTAGUE FINNISTON OLlg/R SIDNEY PLAIL NEUTRONIC REACTOR FUEL ELEMENT AND METHOD OF MANUFACTURE Harold Montague Finniston and Oliver Sidney Plail, London, England, assignors, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Filed Feb. 21, 1950, Ser. No. 145,408

15 Claims. (Cl. 204-193.2)

This invention relates to the production of uranium bodies for use in nuclear fission reactors and particularly to the production of such uranium bodies as comprise a shaped mass of uranium metal enclosed in a protective metallic sheath or envelope.

A form of such a uranium body which has been employed in nuclear reactors consists of a rod of uranium metal enclosed in a sealed can of relatively thin aluminium. To improve contact between the aluminium and the uranium, and so facilitate passage of heat from the uranium, drawing of the can by passage of the canned uranium through a die has been practised. Bonding of the aluminium can to the uranium would result in more satisfactory conduction of heat from the body, but attempts to bond uranium masses to thin aluminium sheaths have not resulted in practical sheathed bodies for operation at temperatures of several hundred degrees centigrade. Thermal contact between uranium masses and unbonded aluminium cans has not been found to be consistently good; and the contact itself is a source of possible failure of the uranium body, since there is a tendency for uranium-aluminium alloy to form at the region of contact at temperatures which are possible in nuclear reactors, and alloy formation has been attended by localised swellings of the can, which are liable to develop into perforations.

In the case of other metals than aluminium which may be employed for sheathing uranium rods, direct bonding of the sheath to the uranium, although advantageous in promoting heat transfer, may not prove desirable or practicable.

It is an object of the invention to provide a uranium body having a protective metallic sheath or envelope in which advantages of direct bonding of the sheath or envelope may be obtained without direct contact of the two.

According to the invention a uranium body for use in a nuclear fission reactor comprises a mass of uranium, a protective sheath or envelope of metal and a thin layer of a metal or alloy between the uranium and the sheath and bonded thereto, the metal of the layer being one which diffuses readily into uranium and the metal of the sheath at elevated temperatures, for example 400 C. to 800 C.

In the case of an aluminium sheath, the metal of the layer preferably has a melting point higher than that of aluminium and does not diffuse rapidly into aluminium or uranium or vice versa at temperatures of operation of nuclear reactors, for example up to 300 C.

Also in accordance with the invention a uranium body comprising a mass of uranium enclosed in a metallic envelope or sheath is produced by applying to a uranium mass a thin coating of a metal which diffuses into uranium and the metal of the sheath read lv at elevated temperatures, for example 400 C. to 800 C. heating the coated uranium mass to an elevated temperature to promote diffusion at the interface, sealing the treated uranium mass in the sheath or envelope and heating the assembly in a tight fitting container to cause the metal of the sheath or envelope and the coating metal to diffuse into each other.

The metal of the layer in general shou'd not form an alloy with the sheath metal of uranium of a melting point lower than the operating temperature of the nuclear reactor, for example lower than 300400 C. It

should diffuse into uranium and the sheath metal, and uranium and the sheath metal diffuse into it, reasonably rapidly at raised temperatures, for example from 400 to 800 C. so that bonding may be effected at such tern: peratures in several hours; but diffusion should not be rapid at nuclear reactor operating temperatures, for example up to 300 C. The alloy layers resulting from the diffusion of the said metal into uranium and the sheath metal should be sufficiently ductile to withstand thermal stresses involved in the production of the uranium bodies and their use in a nuclear reactor. The metal layer shou'd have a low competitive neutron absorption.

The thIn layer of metal may be applied to the clean surface of the uranium mass by electrodeposition, by evaporation of the metal in vacuum or by chemical means. The layer may then be bonded to the uranium by heating the composite mass, in vacuo or in a protective atmosphere, to allow diffusion at the interface. Temperatures between 500 C. and 800 C. have been found suitable for this purpose. An aluminium sheath or other metal sheath may then be cast in position about the mass A bond will be formed between the metal layer and the aluminium or other metal sheath by diffusion across the boundary. Alternatively, a uranium mass with its bonded layer of metal may be provided with a sheath of a uminium or other metal, e.g. magnesium by ordinary canning methods and the sheath drawn down into close contact with the metal layer. A bond between the metal layer and the sheath may then be produced by promoting diffusion across the metal sheath boundary by heating the assembly in a tightly fitting container to a temperature of from 300 to 600 C.

Examples of metals which may be employed for the layer between uranium and sheath metal e.g. aluminium,

are nickel, cobalt and iron. A layer thickness of the order of substantia'ly 0.001 inch or more is desirable for effective bonding with the uranium and the aluminium without the formation of interface compounds or alloys of uranium and aluminium, although since the layer metal constitutes an additional absorber of thermal neutrons it is preferable to adopt a thickness as small as is consistent with the making of a sound bond. Alternative metal or alloys for the sheath are beryl'iurn, zirconium and magnesium, and aluminum-magnesium alloy.

The following is an exampe of a preferred way of carrying the invention into effect as applied to a uranium rod sheathed in aluminium.

Example A uranium rod is attached at one end to mild steel carrier to allow of the rod being treated without being touched by hand. The rod is degreased by immersion in acetone for about one minute and then suspended by the carrier from a central anode bar of an electrolytic cell having two graphite plates arranged on either side as cathodes. The electrolyte employed is'a 10 percent aqueous citric acid solution and electrolysis is allowed to process for one or two minutes to clean the uranium bar. The bar is removed by means of its carrier, rinsed in water drained and introduced into an electrolytic cell suitable for nickel plating the bar being made the cathode. The electrolyte maybe a sulphate bath as ordinarily employedforelectroplatingnickel.- A deposit of 1 1 1 1 0. Ql inch1 or somewhat less is laid on the: uranium bar 1 1 1 1 and; the latter isremoved from the electrolyte, rinsed 1 .1 1

, 1 minutes; It is then immersedin a five percent aqueous 1 L 1 caustic sodasoiution at approximately 60 1G for sub- 1 1 1 stantially :two -minutes,: washedin water, drained and 1 1 immersed: in a nitric acid solution 1 of 40 to 70 percent concentration-for at leastone minute 1 The can is then 1 1 withdrawn allowed :to drain, washed with cold water 1 1 and then hot water and :after draining dried in Z3111 oven .1 1

1 at 100 :C. to i501C; for at least 2O minutes.1 1 .:1:1:1 1 1 .The treated uranium rod is introduced-into the alumin- 1 1 1 iurn; can and :a closure of aluminium is sealed over the 1 :openend as byweldingor soldering- The canned rod 1 1 1 is introduced into a :tightfitting or: split steel sleeve 1 1 graphitised; internallyto: prevent adhesion: of the :sleeve 1 1 1 face to 1 the can :wali. r The 1 assembly 1 is then 1 heated to 1 1 1 1 substantially 500 9 (3.5 forthree hours to promote diffusion" 1 :of the aluminium into the nickel layer and vice VBI'SR: 1 1

Alternatively the nickel film may: he :appliedto 1the1 1 uranium 1 rod by evaporation: in vacuo; 1 1 1 thickness of envelope 2 and layer 3 is greatly exaggerated in the drawing.

We claim:

1. A uranium body for use in a nuclear fission reactor comprising a homogeneous rod of uranium metal enclosed in an envelope of aluminium, wherein a thin metallic layer of higher melting point than aluminium and of relatively low competitive neutron absorbtion between the uranium and the aluminium is bonded to the uranium and to the aluminium of the sheath.

2. A uranium body for use in a nuclear fission reactor comprising a homogeneous rod of uranium metal, an envelope of aluminium enclosing the mass of uranium and a thin layer of nickel between the uranium and the envelope, the nickel being bonded to the uranium and to the aluminium of the sheath.

3. A uranium body for use in a nuclear fission reactor comprising a homogeneous rod of uranium metal, an

1 1 and dried. The plated rod is: heated: invacuo or an 1 1 1 1 atmosphere 1 of argonto: a temperature :of substantially 1 .650 C. forsubstantially twohours to bring :about. (lif 1 1 1 fusion of: meta across the: nickel uraniuminterface; 1 1 1 v A111aluminium can of a size just to admit the uranium= 1 1 rod; is degreased in; trichiorethyiene vapour for 1 five 1 1 minutes :and then 11 in: liquid trichlorethylene :for 1 five 1 1 a nuclearfission reactor, 2 which comprises a 1 homogeneous 1 1 1 1 1 1 1 1 1 1 rod of uranium metal within a protective :metallic en 1 1 1 1 1 :In a manner similar to that: described in the example 1 a uraniummass enclosed in art-aluminium; envelope may 1 1 i bezbonded: to the envelope-by: means of a thin layer of 1 1 z gobalt1er irgn 1 i i Bodies :of the invention are illustrated; in the .accotn': panyingdrawing,;in w hich:: lFig .1 is a perspective =view1of a body, and 1 1 1 1 1 1 -Fig .'2iis a cross-section thereof.

; The; body consists of: a cylindrical; core 1 of 'uranium 1 i 1 1 1 and a: protective envelope 210f aluminium; fberylliurn i 1 or'other suitable metal: as hereinbefore specified, and :a; 1

1 thin layer 3 of 1 nickel; 1 cobalt, 1 iron or the like. 1 The coating metal to diffuse into each other.

which comprises 'aihomogieneous rod of uranium metal; 1 1 1 1 1 1 1 a protective envelope of: metal of: the group consisting 1 1 of aluminium, beryllium, magnesium, and aluminium 1 1 1 1 1 magnesium alloy forthe-uranium and-a thimlayenota 1 1 1 1 1 1 further metal- :of low/competitiveneutronabsorption be- 1 1 tween theuranium and the envelope bondedto the en. 1 1

velopeandithenranium.= 2 .1.:1 11 1 6. A uranium body for use :in a nuclear fission reactor i 1 1 1 .which1comprises a homogeneous rod of uranium:metal, a protective: metallic envelope :for1the. uranium 1 and a: 1 thin: layer. of nickel between the uranium and :tl1e:en velope, the: nickel being bonded to 1 the uranium: and to; 1 'thernetal ofthesheatinww 1 1 1' 7. Auranium body-for use in whichcomprises a homogeneous rod of uranium metal,- 1 a protective metallic envelope .for the uranium 1 and a: 1 1 thin layer: of cobalt between 1 the uranium and :the: en-- 1 1 velope, the: cobalt being bonded i to 1 theuranium. and to- 'the'metalofthe'sheathr: 2 1 f 1 1 8.- A. uranium body for use in which comprises a homogeneous rod :of uranium; metal, 1 I a protective: metallic envelope tor the: uranium 1 and a 1 1 1 I thin l ayer of iron betweenthe uranium and. the envelope, 1

the: iron being bonded to the uranium; and to the metal: 1 1 0f the'shea'th 1 1 1 1 a nuclear fission reactor 1 1 1 a nuclear fission reactor 1 A; method of producing. a1 uranium. body for use in 1 r 1 velope', the method 1 includin g the steps of i applying to the ing into uranium and the metal of the envelope at an 1 1 elevated temperature of substantially 4G01 C.-to 800 G 1 1 iheating: the: coated uranium in :nonsreaetive surroundings 1 to such elevated ternpcratureto; cause diifusion 'of the 1 1 1 coating metal into the uranium, introducing the treated 1 1 1 i uranium into a preformed enveiopez, closing and sealing 1 1 the envelope and heating: the assem=bly :to substantially 1 1 1 2 500 (3. for several. hours while contained in a night-1 1 1 1 ifitting container to eausei the metal of the envelope and 1 the coating metal to diffuse. into each other. 1 1 1 1 1 1 11 1 1 1 10. A method of producing a uranium body comprising a homogeneous rod of uranium metal within a protective envelope of aluminium which comprises applying to the uranium a thin metallic coating heating the coated uranium in non-reactive surroundings to 400 C. to 800 C. to cause diffusion of the coating metal into the uranium, applying a preformed envelope of aluminium to the treated uranium, and heating the assembly to substantially 500 C. for several hours while contained in a tight fitting container to cause the aluminium and the 11. A method according to claim 10 wherein the peat ing metal is nickel.

12. A method according to claim 10 wherein the coating metal is cobalt.

13. A method according to claim 10 wherein the coating metal is iron.

14. A method according to claim 9 wherein the coated uranium is heated to the elevated temperature for suhl l l stantially two hours.

15. A method according to claim 9 wherein the assembly is heated for substantially three hours.

References Cited in the file of this patent UNITED STATES PATENTS 2,266,330 Nachtman Dec. 16, 1941 FOREIGN PATENTS 1 861,390 France n, Oct. 28, 1940 1 

1. A URANIUM BODY FOR USE IN A NUCLEAR FISSION REACTOR COMPRISING A HOMOGENEOUS ROD OF URANIUM METAL ENCLOSED IN AN ENVELOPE OF ALUMINIUM, WHEREIN A THIN METALLIC LAYER OF HIGHER MELTING POINT THAN ALUMINIUM AND OF RELATIVELY LOW COMPETITIVE NEUTRON ABSORBTION BETWEEN THE URANIUM AND THE ALUMINIUM IS BONDED TO THE URANIUM AND TO THE ALUMINIUM OF THE SHEATH. 